s how and why upper layers of Earth are mobile, to examine internal structure of Earth and plate t ernal structure of Earth Earth is layered knowledge of layering is recent (late 1800s) prior to that, only knew interior must be hot (volcanoes) mers calculated mass from radius and gravitational for 2,000 years) mean density ~ 5500 kg/m 3 , but surface rocks ~2300 k therefore, density gradient exists
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To discuss how and why upper layers of Earth are mobile, need to examine internal structure of Earth and plate tectonics internal structure of Earth Earth.
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to discuss how and why upper layers of Earth are mobile,need to examine internal structure of Earth and plate tectonics
internal structure of EarthEarth is layered
knowledge of layering is recent (late 1800s);prior to that, only knew interior must be hot
(volcanoes)
astronomers calculated mass from radius and gravitational constant(known for 2,000 years)
mean density ~ 5500 kg/m3, but surface rocks ~2300 kg/m3
therefore, density gradient exists
Mantle Pie Section and Seismic Velocities
Mantle Subdivisions
assumed density increased smoothly with depth due to increaseof pressure with depth
estimate for center of Earth was 10,000-12,000 kg/m3 (not bad!)
breakthrough came with idea that seismic waves generated byearthquakes could travel through the entire Earth and
be recorded elsewhere on the surface: seismology
differentiation of Earthearly in its history
travel paths of seismic waves generated by earthquakes;directly through the earth or reflected by discontinuities
different behavior of P and S waves led to idea of liquid layer in interior
• changes in chemical composition (compositional changes)• changes in mineral structures (phase changes)
what changes occur where is a large area of research………cannot make direct observation!draw from geochemistry, mineral physics, meteoritics,
igneous petrology, seismology
crust: felsic (shallow) to maficmantle: ultramafic (peridotite)outer core: liquid iron alloyinner core: solid iron alloy
crust/mantle: Mohorovicic discontinuity (Moho)--compositionalmantle/core: Gutenberg discontinuity--compositionalinner/outer core: phase (liquid to solid)400 km discontinuity: phase (olivine to spinel structure)670 km discontinuity: phase (spinel structure to perovskite)
crust and mantle(remember that they are distinguished on the basis of
their physical properties)
how do we know what is at depth?electrical conductivity: identifies partial meltsexposed deep crust: occurs in mountain belts; 50 km originallygeochemistry and elemental abundances: tell range of compositiongravity anomalies: identifies density differenceslithospheric flexure: constrains rheologymagnetic anomalies: shows distribution of subsurface rocksmineral physics: measures seismic velocities in rock samplesophiolites: represents oceanic lithospherexenoliths in volcanic rocks: represents upper mantleseismic reflection: identifies changes in lithologyseismic refraction: defines velocities of seismic waves at depthseismic tomography: permits 3D visualization
obvious from space that Earth has two fundamentally differentphysiographic features: oceans (71%) and continents (29%)
magnetic anomalies allow dating of oceanic crust… …for basalts intensity of remanent magnetism > induced
anomalies will vary with latitude and ridge orientation
if oceanic crust acquires its magnetism at high latitudes… magnetization vector dips steeply…
in northern latitudes……dips steeply north for normal…points steeply up and south for reversed
…closer to equator, magnetization vector not as steep…at equator, magnetization vector horizontal
negative anomaly coincides with normal blocks
as a consequence of seafloor spreading (and subduction),oceanic crust is < 200 Ma old (with exception of ophiolites)
note pattern of increasing age away from ridges
continental crust
• 5-10 times thicker than oceanic crust (40-70 km thick)• average chemical composition is similar to granodiorite• heterogeneous vertically and laterally• wide range of ages
most elements forming continental crust migrated from Earth’sinterior during Archean (3.8-2.5 Ga): differentiation
Earth was too hot to form permanent crust prior to 3.8 Ga;surface likely convecting ultramafic material
at ~ 3.8 Ga, interior of Earth cooled enough to allow a crust to form; only partial melting occurred (minerals melt at low temperature)
subsequent fractionation and crystallization led to variations incomposition from mafic to silicic
mantle
general composition of peridotitefrom seismic velocities, xenolith compositions
seismic tomography (similar to CAT scan of Earth)suggest inhomogeneous in 3 dimensions
--variations in composition? temperature? both?
composition: extraction of basaltic magma to produce oceanic crust
lithosphere and asthenospheredistinguished by response to stress (their “strength”)---not by seismic discontinuitiesthermal boundary (more in a minute)
lithosphere first proposed to explain isostasy--response of Earth’s surface to vertical loads
(growths of glaciers, formation of islands)
upper most rheologic layer of Earth (lithos-rock):exhibits flexural rigidity on geological time scales
(resistance to bending)
steel: high flexural rigidity rubber: low flexural rigidity
over long time periods, lithosphere does flow (more later)
lithosphere moves as a coherent entity: plate• contains crust and uppermost mantle• base is the 1280°C isotherm (thermal boundary) at this temperature, peridotite weakens due
to easy deformation of olivine• base is not fixed depth; depth of 1280°C isotherm varies below ridges, temperatures high (lithosphere thin-few km) below cratons, temperatures low (lithosphere thick-150 km)
asthenosphere behaves like a viscous fluid on geological time scales
• layer of mantle below lithosphere• composed of predominantly solid, although, weak rock• low flexural rigidity• material flow (crystal plastic flow, diffusion): convection• low velocity zone exists in asthenosphere below oceans
(partial melting? rheology of olivine?)• base of asthenosphere problematic: 400 km; 670 km; core?
(layered convection? whole mantle convection?)
lithosphere “strong” asthenosphere “weak”
The layered Earth and ophiolites
websites from which images are drawn:
sources: Kearey, P. and F. Vine, 1996, Global tectonics, second edition, Blackwell